Direct drive for a subscriber line differential ringing signal having a DC offset

ABSTRACT

A method of generating a subscriber line differential ringing signal with a DC component includes providing a time-varying supply level, W(t), having a plurality of non-equidistantly spaced critical points along a folding line. W(t) is coupled to the tip line while coupling an alternate source to the ring line in response to a first critical point. W(t) is coupled to the ring line while coupling the alternate source to the tip line in response to a second critical point. An apparatus for generating the ringing signal includes a power supply providing a supply level W(t)=|f(t)−C|+C+D, wherein D is a power supply offset, wherein C≠0. When W(t)≦K a signal processor controls a linefeed driver to toggle between 1) coupling W(t) to the tip line while coupling the ring line to an alternate supply, V ALT (t), and 2) coupling W(t) to the ring line while coupling the tip line to V ALT (t), wherein K is a pre-determined switching threshold.

FIELD OF THE INVENTION

This invention relates to the field of telecommunications. Inparticular, this invention is drawn to subscriber line interfacecircuitry.

BACKGROUND OF THE INVENTION

Subscriber line interface circuits are typically found in the centraloffice exchange of a telecommunications network. A subscriber lineinterface circuit (SLIC) provides a communications interface between thedigital switching network of a central office and an analog subscriberline. The subscriber line comprises a tip line and a ring line. Theanalog subscriber line connects the SLIC to a subscriber station orsubscriber equipment such as a telephone at a location remote from thecentral office exchange.

In conjunction with the subscriber equipment, the tip and ring linesform a subscriber loop. The SLIC communicates both data and controlsignals with the subscriber equipment. Control signals tend to requiresignificantly greater voltages and currents than data signals (i.e.,voiceband data) on the subscriber loop.

One subscriber equipment control signal requiring relatively highvoltages and current is a ringing signal. Typically, the SLIC provides asinusoidal or trapezoidal ringing signal to the subscriber equipment.Various approaches to ringing signal generation include unbalanced andbalanced ringing.

Unbalanced ringing is accomplished by holding one of the tip and ringlines at a pre-determined voltage level (e.g., ground) while providingthe other line with the ringing signal. One disadvantage of unbalancedringing is that the maximum DC operating voltage for the SLIC must bethe greater of: i) any DC offset plus the peak ringing voltage, or ii)the peak-to-peak ringing voltage. Such voltage levels tend to limitfabrication of a ringing signal generator to either high voltageintegrated circuits or discrete components.

Balanced ringing provides a ringing signal to each of the tip and ringlines. Typically the AC component of the ringing signal applied to bothlines is identical with the exception of a 180° phase shift. As aresult, balanced ringing requires a significantly lower DC operatingvoltage than unbalanced ringing. One disadvantage of traditionalbalanced ringing is that AC matched ringing signals with a relativephase shift of 180° must be generated.

SUMMARY OF THE INVENTION

In view of limitations of known systems and methods, a method ofgenerating a subscriber line ringing signal having a DC offset areprovided.

A method of generating a differential ringing signal with a DC componentbetween a tip and a ring line of a subscriber line includes the step ofproviding a time-varying supply level, W(t), having a plurality ofcritical points along a folding line, wherein the critical points aresubstantially not equidistant. W(t) is coupled to the tip line whilecoupling an alternate source to the ring line in response to a firstcritical point. W(t) is coupled to the ring line while coupling thealternate source to the tip line in response to a second critical point.

An apparatus for generating a subscriber line ringing signal includes asignal processor, a linefeed driver, and a power supply providing atime-varying supply level W(t)=|f(t)−C|+C+D, wherein D is a power supplyoffset, wherein C≠0. When W(t)≦K a signal processor controls a linefeeddriver to toggle between 1) coupling W(t) to a tip line while coupling aring line to an alternate supply, V_(ALT)(t), and 2) coupling W(t) tothe ring line while coupling the tip line to V_(ALT)(t), wherein K is apre-determined switching threshold.

In various embodiments, the differential ringing signal resembles atrapezoidal or a sinusoidal waveform and W(t) resembles one of afull-wave rectified sinusoidal or trapezoidal waveform.

Other features and advantages of the present invention will be apparentfrom the accompanying drawings and from the detailed description thatfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 illustrates one embodiment of a central office exchange includinga subscriber line interface circuit (SLIC) coupling subscriber equipmentto a digital switching system network.

FIG. 2 illustrates one embodiment of a SLIC architecture.

FIG. 3 illustrates one embodiment of ringing signal components forgeneration of an unbalanced differential ringing signal.

FIG. 4 illustrates one embodiment of ringing signal components forgeneration of a balanced differential ringing signal.

FIG. 5 illustrates one embodiment of a rectified waveform anddecomposition of the rectified waveform into matched ringing signalcomponents with a 180° relative phase difference.

FIG. 6 illustrates one embodiment of a method for generating matchedringing signal components with a 180° phase difference.

FIG. 7 illustrates application of the method of FIG. 6 to one embodimentof a rectified waveform for generating the differential ringing signal.

FIG. 8 illustrates prior art generation of a differential ringing signalusing linear amplifiers.

FIG. 9 illustrates one embodiment of an apparatus generating adifferential ringing signal through controlled switching.

FIG. 10 illustrates one embodiment of a linefeed driver.

FIG. 11 illustrates one embodiment of a rectified waveform forgenerating the differential ringing signal with a DC offset.

FIG. 12 illustrates a method of toggling switching circuitry in responseto power supply inflection points along a folding line to generate adifferential ringing signal.

FIG. 13 illustrates a plurality of rectified waveforms.

FIG. 14 contrasts generation of a differential ringing signal through anintervening linear amplifier versus direct driving of the subscriberline.

DETAILED DESCRIPTION

FIG. 1 illustrates functional elements or one embodiment of a subscriberline interface circuit (SLIC) 110 typically associated with plain oldtelephone services (POTS) telephone lines. The subscriber line interfacecircuit (SLIC) provides an interface between the digital switchingnetwork 120 of a local telephone company central exchange and asubscriber loop 132 including subscriber equipment 130.

The subscriber loop 132 is typically used for communicating analog datasignals (e.g., voiceband communications) as well as subscriber loop“handshaking” or control signals. The analog data signals are typicallyon the order of 1 volt peak-to-peak (i.e., “small signal”). Thesubscriber loop control signals typically consist of a 48 V DC offsetand an AC signal of 40–140 Vrms (i.e., “large signal”). The subscriberloop state is often specified in terms of the tip 180 and ring 190 linesof the subscriber loop.

The SLIC is expected to perform a number of functions often collectivelyreferred to as the BORSCHT requirements. BORSCHT is an acronym for“battery feed,” “overvoltage protection,” “ring,” “supervision,”“codec,” “hybrid,” and “test.”

The SLIC provides power to the subscriber equipment 130 using thebattery feed function. The overvoltage protection function serves toprotect the central office circuitry against voltage transients that mayoccur on the subscriber loop 132. The ring function enables the SLIC tosignal the subscriber equipment 130. In one embodiment, subscriberequipment 130 is a telephone. Thus, the ring function enables the SLICto ring the telephone.

The supervision function enables the SLIC to detect service requestssuch as when the caller places the subscriber equipment off-hook toinitiate or receive a call. The supervision function is also used tosupervise calls in progress and to detect dialing input signals.

The hybrid function provides a conversion from two-wire signaling tofour-wire signaling. The SLIC includes a codec to convert the four-wireanalog voiceband data signal into serial digital codes suitable fortransmission by the digital switching network 120. In one embodiment,pulse code modulation is used to encode the voiceband data. The SLICalso typically provides a means to test for or to indicate faults thatmay exist in the subscriber loop or the SLIC itself.

The codec function has relatively low power requirements and can beimplemented in a low voltage integrated circuit operating in the rangeof approximately 5 volts or less. The battery feed and supervisioncircuitry typically operate in the range of 40–75 volts. In someimplementations the ringing function is handled by the same circuitry asthe battery feed and supervision circuitry. In other implementations,the ringing function is performed by higher voltage ringing circuitry(e.g., 75–150 V_(rms)). Thus depending upon implementation, the ringingfunction as well as the overvoltage protection function may beassociated with circuitry having greater voltage or current operatingrequirements than the other circuitry.

FIG. 2 illustrates one embodiment of a SLIC 200 wherein the BORSCHTfunctions have been distributed between a signal processor 210 and alinefeed driver 220. Signal processor 210 is responsible for at leastthe ring control, supervision, codec, and hybrid functions. Signalprocessor 210 controls and interprets the large signal subscriber loopcontrol signals as well as handling the small signal analog voicebanddata and the digital voiceband data.

In one embodiment, signal processor 210 is an integrated circuit. Theintegrated circuit includes sense inputs for sensing the tip and ringlines of the subscriber loop. The integrated circuit generatessubscriber loop linefeed driver control signals in response to thesensed signals. In one embodiment, the linefeed driver does not residewithin the integrated circuit or within the same integrated circuitpackage as the signal processor 210. In alternative embodiments, thesignal processor may reside within the same integrated circuit packageas at least a portion of the linefeed driver.

Signal processor 210 receives subscriber loop state information fromlinefeed driver 220 as indicated by tip/ring sense 222. This informationis used to generate control signals for linefeed driver 220 as indicatedby linefeed driver control 212. The voiceband 230 signal is used forbi-directional communication of the analog voiceband data betweenlinefeed driver 220 and signal processor 210.

Signal processor 210 includes a digital interface 219. The digitalinterface includes a digital voiceband interface 216 for communicatingdigital voiceband data 218 between the signal processor and the digitalswitching network. In one embodiment, the digital interface includes aprocessor interface 214 to enable programmatic control of the signalprocessor 210. The processor interface effectively enables providingprocessor control data 215 to obtain programmatic or dynamic control ofbattery control, battery feed state control, voiceband dataamplification and level shifting, longitudinal balance, ringingcurrents, and other subscriber loop control parameters as well assetting thresholds such as ring trip detection and off-hook detection.

The digital voiceband data 218 is coupled to a digital codec interface242 of signal processor 210 for bi-directional communication of thedigital voiceband data between the codec 240 of the signal processor andthe digital switching network. The analog voiceband data 230 is coupledto an analog codec interface 244 of signal processor 210 forbi-directional communication of the analog voiceband data between thecodec and the linefeed driver.

Linefeed driver 220 maintains responsibility for battery feed to tip 280and ring 290. Overvoltage protection is not explicitly illustrated.Overvoltage protection can be provided by fuses incorporated intolinefeed driver 220, if desired. Linefeed driver 220 includes sensecircuitry to provide signal processor 210 with sensed subscriber loopoperating parameters as indicated by tip/ring sense 222. Signalprocessor 210 performs any necessary processing on the sensed parametersin order to determine the operational state of the subscriber loop. Forexample, differences or sums of sensed voltages and currents areperformed as necessary by signal processor 210 rather than linefeeddriver 220. Thus common mode and differential mode components (e.g.,voltage and current) of the subscriber loop are calculated by the signalprocessor rather than the linefeed driver.

Linefeed driver 220 modifies the large signal tip and ring operatingconditions in response to linefeed driver control 212 provided by signalprocessor 210. This arrangement enables the signal processor to performprocessing as needed to handle the majority of the BORSCHT functions.For example, the supervisory functions of ring trip, ground key, andoff-hook detection can be determined by signal processor 210 based onoperating parameters provided by tip/ring sense 222.

Power supply 250 provides the voltage V_(BAT) to the linefeed driver fordriving the tip and ring lines. In one embodiment, power supply 250 is aswitching power supply controlled by signal processor 210 to provide theappropriate DC output voltage.

The tip and ring lines form a differential pair for communicatingvoiceband and handshaking signals to the subscriber equipment. Ringingof the subscriber equipment is accomplished using a differential ringingsignal applied to the tip and ring lines. The differential ringingsignal is thus composed of a ringing signal component applied to the tipline and a ringing signal component applied to the ring line. Thedifferential ringing signal is classified as “unbalanced” or “balanced”depending upon the characteristics of the ringing signal components.

The differential ringing signal components may be described in terms oftime varying currents or time varying voltages. For purposes ofillustration, a level of abstraction regarding the assignment of thesecomponents to the tip or ring lines is introduced. Namely, the examplesrefer to a first line ringing signal component L1(t) and a second lineringing signal component L2(t), rather than a tip or ring line ringingsignal component. The assignment of L1(t) and L2(t) to tip and ringlines depends upon the desired polarity or phase for the differentialringing signal.

FIG. 3 illustrates embodiments of subscriber line waveforms forunbalanced ringing. Waveform 3(a) represents the first line voltage,L1(t). Waveform 3(b) represents the second line voltage, L2(t). Waveform3(c) represents the difference ΔL(t)=L1(t)−L2(t) between the first andsecond lines.

The first line is set to a pre-determined level such that L1(t) isconstant. In the illustrated embodiment, the first line is coupled to aground node such that L1(t)=0 as indicated. The second line has an ACcomponent and an optional DC component. For example, if L2(t)=−48+127sin(ωt), the DC component is −48 and the AC component is 127 sin(ωt).The differential ringing signal is L1(t)−L2(t)=48−127 sin(ωt) asindicated in waveform 3(c).

The second line peak-to-peak voltage (L2(t)_(PP) _(—) _(U)) is the sameas the differential peak-to-peak voltage (ΔL(t)_(PP) _(—) _(U)) betweentip and ring such that the entire differential ringing signal swing iscarried on only one line. Unbalanced ringing is thus characterized byusing only one of the tip and ring lines to carry the entire signalswing.

In contrast, “balanced ringing” implies applying matched AC signals(with a relative phase difference) to the tip and ring lines such thateach line is responsible for half the swing range of the resultingdifferential ringing signal. Traditionally, the individual line ringingsignal components have the property that L1(t)=−L2(t) such that thecommon mode component is zero

$\left( {{i.e.},{\frac{{{L1}(t)} + {{L2}(t)}}{2} = 0}} \right).$

Typically, L1(t) and L2(t) are periodic and exhibit the properties

${{{\int_{0}^{T}{{L1}(t)}} - {\frac{1}{T}\ {\int_{0}^{T}{{L1}(t)}}}} = 0}\ $and

${{{\int_{0}^{T}{{L2}(t)}} - {\frac{1}{T}\ {\int_{0}^{T}{{L2}(t)}}}} = 0}\ $over the period T, where

${\frac{1}{T}{\int_{0}^{T}{{L1}(t)}}},{\frac{1}{T}{\int_{0}^{T}{{L2}(t)}}}$represent the mean or DC components of L1(t) and L2(t).

FIG. 4 illustrates subscriber line waveforms for balanced ringing. Thetip and ring lines have the same AC component with a relative phaseshift of 180°. Waveform 4(a) represents the first line ringing signalcomponent, L1(t)=63.5 sin(ωt). Waveform 4(b) represents the second lineringing signal component, L2(t)=63.5 sin(ωt+π) such that the ACcomponents of the tip and ring lines are phase shifted 180° relative toeach other. Waveform 4(c) represents the differential ringing signal,ΔL(t)=L1(t)−L2(t). Substituting the ringing signal component valuesyields ΔL(t)=63.5 sin(ωt)−63.5(ωt+π). Given that sin(ωt+π)=−sin(ωt), theexpression can be simplified to ΔL(t)=127(ωt).

Comparing FIGS. 3 and 4, the AC component of ΔL(t) is the same forbalanced and unbalanced ringing such that the resulting peak-to-peakringing signal provided to the subscriber equipment is the same(ΔL(t)_(PP) _(—) _(U)=ΔL(t)_(PP) _(—) _(B)). The maximum peak-to-peakvalue of any ringing signal component contributing to a balanceddifferential ringing signal, however, needs only to be half of themaximum peak-to-peak value of any ringing signal component contributingto an unbalanced differential ringing signal (i.e., L1(t)_(PP) _(—)_(B)=L2(t)_(PP) _(—) _(B)=½L2(t)_(PP) _(—) _(U)).

Thus, a signal of only half the swing required for unbalanced ringingcan be used on each of the tip and ring lines for balanced ringing. Thedisadvantage of balanced ringing, however, is that AC matched ringingsignals with a 180° phase difference must be generated.

FIG. 5 illustrates an alternate waveform 5(a) from which individualringing signal components of the differential ringing signal can beextracted. Alternative waveform 5(a) is a full-wave rectified sinusoid.In the illustrated embodiment, the rectified sinusoid corresponds to asinusoid of period T folded in half about a folding point defined by themean of the sinusoid. If there is no DC offset, W=|W_(p) sin(ωt)|. Theperiod of the resulting waveform is T/2. The full-wave rectifiedsinusoid is equivalent to the sum of two half-wave rectified sinusoidsθ₁, θ₂ that are 180° phase shifted from each other as indicated inwaveforms (b) and (c). The period of waveforms (b) and (c) is T.

FIG. 6 illustrates one method of generating a differential ringingsignal from a rectified or folded waveform. In step 610, a rectifiedwaveform of period T/2 is applied to the tip line while the ring line ismaintained at a pre-determined supply level for the duration T/2. Instep 620, the rectified waveform is applied to the ring line while thetip line is maintained at a pre-determined supply level for the durationT/2. Each line thus alternately receives a time varying waveform for aduration T/2 and a pre-determined supply level for the duration T/2. Theprocess is repeated for the duration of ringing by toggling applicationof the rectified waveform between a selected one of the tip and ringlines while holding the other line at the pre-determined supply level.

In one embodiment, the pre-determined supply level is ground (e.g.,approximately 0 volts). In one embodiment, the rectified waveform is afull-wave rectified sinusoid. Alternatively, the rectified waveform is atrapezoidal waveform.

Referring to FIG. 5, rectified waveforms such as 5(a) (full-wave) or5(b) (half-wave) are characterized by a plurality of critical pointsthat lie on a folding line. A critical point for a waveform, W(t),exists everywhere W(t) is not differentiable as well as where the firstderivative of W(t) is zero (i.e., d/dtW(t)=0 or d/dtW(t) does notexist). For example, waveform 5(a) has critical points including 512,514, and 516. Waveform 5(b) has critical points at 522, 524, and 526. Inone embodiment, the toggling occurs at or near the critical points ofthe rectified waveform.

Given that the critical points are closer to the folding point than theremainder of the waveform, a threshold detector may be used to identifycritical points and determine when to toggle. Thus toggling occurs whenthe rectified waveform falls below a pre-determined switching threshold,K. The threshold detector may be used in conjunction with adifferentiator or other mechanism to ensure that toggling occurs onlyonce for each critical point.

FIG. 7 illustrates the application of the method of FIG. 6 to first andsecond lines of a subscriber line comprising tip and ring lines.Waveform 7(a) (W(t)) resembles a full-wave rectified sine wave. W(t) isapplied to the first line while holding the second line at apre-determined supply level of 0 volts (i.e., ground). When W(t)approaches critical point 710 near zero, a toggling of the applicationof W(t) and the pre-determined supply level occurs. In particular, W(t)is now applied to the second line while the first line is grounded. WhenW(t) subsequently approaches critical point 720, another togglingoccurs. In particular, W(t) is applied to the first line while thesecond line is grounded.

Waveforms 7(b) and 7(c) represent the first (L1(t)) and second (L2(t))line signals generated as a result of the toggling. The resulting firstand second line signals approximate half-wave rectified sinusoids in theillustrated embodiment. The resulting differential ringing signalΔL(t)=L1(t)−L2(t) is illustrated as waveform 7(d). The frequency of thedifferential ringing signal ΔL(t) is one-half that of W(t) in theillustrated embodiment. In one embodiment, the differential ringingsignal ΔL(t) has substantially no DC component.

The AC components of L1(t) and L2(t) are identical with the exception ofa relative 180° phase shift. Thus the amplitude of the AC portion of theringing signal component applied to each line need only be half thatrequired of an unbalanced ringing signal to achieve the samedifferential result. Moreover, waveforms (b) and (c) were readilyextracted from waveform (a) rather than being independently generatedsuch that the difficulties of generating identical sinusoids or otherwaveforms 180° out of phase is substantially avoided.

In the illustrated embodiment, L2(t)=L1(t+T/2) where T is the period ofboth L1(t) and L2(t). At least one of the ringing signal components istime varying over the interval between any two critical points. Thus fort∈(0, T/2) there is some point t=z for which the derivative of at leastone of L1(t) and L2(t) is non-zero. In other words, there exists somez∈(0, T/2) such that either

$\left. {\frac{\mathbb{d}\;}{\mathbb{d}t}{{L1}(t)}} \middle| {}_{t = z}{\neq {0\mspace{14mu}{or}\mspace{14mu}\frac{\mathbb{d}\;}{\mathbb{d}t}{{L2}(t)}}} \middle| {}_{t = z}{\neq 0.} \right.$

Waveforms such as W(t) are typically already available from anintermediate stage of a SLIC power supply that provides a variableV_(BAT). Some SLIC power supplies generate an appropriate V_(BAT) forthe particular operational mode of the SLIC. V_(BAT) may be generatedthrough rectification of an AC signal. Alternatively, V_(BAT) may begenerated through processes other than rectification.

A switched mode power supply, for example, may generate a waveform thatis filtered to produce a V_(BAT) with substantially no AC component.This DC V_(BAT) is then used to power linear amplifiers to produce theappropriate differential ringing signal. This approach is inefficient,however, due to filter conversion and the power inefficiency of linearamplification. Direct driving of the subscriber line may be achieved byindependently driving the tip and ring lines with ringing signalcomponents derived from an existing power supply waveform.

FIG. 8 illustrates prior art generation of a ringing signal using linearamplification. Supply voltage V_(BAT) 820 powers ringing signalgenerator 810. Low power ringing signal components (e.g., sinusoidalwaveforms 812, 814 produced by oscillator 802) are applied to eachlinear amplifier 830. Each linear amplifier produces a high power ACringing signal component that contributes to the differential ringingsignal 832 for driving the subscriber line 850. In the illustratedembodiment, the linear amplifiers 830 are class A amplifiers.

Class A amplifiers are linear amplifiers biased such that the activedevices conduct for the full signal swing of the input waveform. Class Aamplifiers tend to have relatively high quiescent power consumption andlow power efficiency. The area 834 between the supply level and theringing signal represents power consumed by the linear amplifiers togenerate the ringing signal components. This power is effectivelywasted.

Some SLIC architectures incorporate a DC-DC converter as the powersupply for generation of the appropriate V_(BAT) used to drive thesubscriber line (see FIG. 2). The full-wave rectified sinusoidalwaveform illustrated as waveform 5(a) or a waveform approximating such afull-wave rectified sine wave (without actual rectification) may beprovided by the power supply.

FIG. 9 illustrates an alternative differential ringing signal generator.Application of the time-varying supply level provided by the powersupply is toggled between the tip and ring lines to permit directdriving of the subscriber line from the power supply. The steps ofconverting the supply level to DC and using the DC supply level toinefficiently linearly amplify ringing signal components is thuseliminated.

SLIC 900 includes signal processor 910 and linefeed driver 920. Signalprocessor 910 includes a power supply control 914 and a switch control916. Power supply control 914 provides the appropriate signals to causethe power supply 930 to generate an output waveform suitable fordecomposition into the differential ringing signal components.

In one embodiment, the power supply control provides digital values tothe power supply 930 and the modulation duty cycle or pulse width of thepower supply is varied accordingly. In the illustrated embodiment, powersupply control 914 provides a small rectified sinusoidal signal 942 toswitching power supply 930. The term “rectified sinusoidal” refers tothe appearance of the waveform. Actual generation of the signal 942 maybe accomplished through rectification or other methods.

Comparator 940 generates a waveform having its pulse width modulated inaccordance with signal 942. In the illustrated embodiment, comparator940 compares signal 942 with a sawtooth waveform 944 to generate a pulsewidth modulated signal 946.

The pulse width modulated signal 946 causes switching circuitry 950 toswitch a much larger supply level, V_(P), thus generating an amplifiedpulse width modulated signal 952 suitable for driving a subscriber line.The amplified pulse width modulated signal 952 is filtered by a low passfilter 960 to produce a filtered output 962 having an output waveformthat is substantially similar to the small signal full-wave rectifiedsinusoid input 942.

The power supply output may be further filtered through an output filter964 to generate the power supply output, V_(BAT) 966. In one embodimentoutput filter 964 provides capacitive filtration to regulate the powersupply output.

The output of the power supply may be unfiltered during ringing. In oneembodiment, the ringing frequency is low enough such that the timeconstant of the capacitive output filter is incapable of performing anysignificant regulation near ringing frequencies. Thus as a result of thelow ringing frequency, the output of the power supply exhibits a“rectified” waveform rather than a substantially constant supply leveleven after filtration.

The small signal input 942 is varied to produce the appropriate V_(BAT)for each of the different operational states of the SLIC (e.g., on-hook,off-hook, ringing). The output filter 964 is designed to produce asubstantially constant V_(BAT) 966 for higher frequency input signals942. For input signals having a frequency near the nominal ringingfrequency, however, output filter 964 is significantly less effective atregulating the power supply output 966 such that V_(BAT) is an amplifiedversion of the small signal input 942. In one embodiment, thedifferential ringing frequency is approximately 15–50 Hz.

The power supply output 966 is provided to signal processor 910 and thelinefeed driver 920. The signal processor provides linefeed drivercontrol signals 912 to the linefeed driver. In one embodiment, thelinefeed driver operates as a linear amplifier when the SLIC is in anoff-hook or on-hook operational state. When in the ringing state, thelinefeed driver is operated as a plurality of switches. The linefeeddriver is thus modeled as a plurality of switches or multiplexers 922,924 during ringing.

Switch control 916 senses the level of the power supply output 966 andprovides the linefeed driver 920 with the appropriate control signals toselectively couple the tip 980 and ring 990 lines to either the powersupply or an alternate source. In this case, the alternate source isground 926 for both tip and ring. In one embodiment, switch control 916comprises a threshold detector for identifying the proximity of criticalpoints using a threshold value for V_(BAT). Switch control 916 mayinclude additional components such as a differentiator for more accuratetoggling of multiplexers 922 and 924.

In the illustrated embodiment, the power supply output is a periodicwaveform with a period of T/2 during the ringing state. When the powersupply output falls below a pre-determined switching threshold, K,switch control 916 causes tip multiplexer 922 to couple the tip line tothe power supply output 966. After a time period of T/2 has elapsed(i.e., the next time the power supply output falls below thepre-determined switching threshold), switch control 916 causes tipmultiplexer 922 to couple the tip line to the alternate source (e.g.,ground 926). The resulting waveform appearing on the tip line thus has aperiod of T and substantially resembles a half-wave rectified sinusoid.

The switch control similarly causes the ring multiplexer 924 to switchbetween the alternate source (e.g., ground 926) and the power supplyoutput 966. The AC components of the resulting waveforms appearing onthe tip and ring lines have approximately a 180° relative phasedifference. The resulting differential ringing signal (e.g.,L1(t)−L2(t)) is substantially sinusoidal and has a period of T (i.e.,frequency=1/T).

In one embodiment, the tip and ring multiplexers switch at approximatelythe same time such that the control signal for one multiplexer is simplyan inverted version of the control signal for the other multiplexer. Inan alternate embodiment, the multiplexers may be switched at slightlydifferent times for waveform shaping purposes in order to achieve adifferential ringing signal that better approximates a sinusoid.

Although there may be some losses within SLIC 900, the linefeed drivermultiplexers effectively switch the power supply output 966 to directlydrive the subscriber line without intervening linear amplifiers.Accordingly, with the exception of the negligible losses of the SLICswitch control and multiplexers, the efficiency of the ringing signalgeneration is limited only by the efficiency of the switching powersupply 930.

FIG. 14 contrasts the prior art differential ringing signal with thedirectly driven differential ringing signal. Waveform (a) illustratesthe losses (region 1410) attributable to attempting to drive thesubscriber line using a fixed V_(BAT) that serves as the amplificationpower rail when intervening linear amplifiers are used. Waveforms (b)and (c) illustrate the ringing signal components L1(t) and L2(t),respectively, that are derived from V_(BAT) in a direct drive approachas a result of toggling between the power supply and ground at selectedswitching points (e.g., 1420). Each component is substantially identicalto V_(BAT) during alternating switching cycles.

The resulting differential ringing signal illustrated as waveform (d) issimilarly identical to V_(BAT) during alternating switching cycles. Forevery other switching cycle, waveform (d) is identical to −V_(BAT).Given that L2(t)=V_(BAT) during this portion of the switching cycle, itfollows that −L2(t)=−V_(BAT). This is a function of the derivation ofthe differential signal. A separate −V_(BAT) is obviously not required.The −V_(BAT) appearing in the differential ringing signal of waveform(d) is the result of coupling V_(BAT) to L2 at switch point 1430 and thedifferential ringing signal is derived from an inverted L2(t).

FIG. 10 illustrates one embodiment of a linefeed driver suitable fordriving the subscriber line using either a linear mode or a switch mode.The linefeed driver includes a voice data portion 1020, sense portion1030, and power portion 1040. Voice data portion 1020 provides voicebanddata from the subscriber line to the signal processor. Voice datacommunicated from the signal processor to the subscriber line may besuperimposed on at least some of the control signals I1–I4 controllingthe power portion of the linefeed driver. Sense portion 1030 allows thesignal processor to monitor the tip and ring line conditions, forexample, to detect when subscriber equipment is on-hook or off-hook aswell for ensuring the appropriate power levels are provided to tip 1080and ring 1090. The power portion 1040 includes a plurality oftransistors Q1–Q6 that drive the tip and ring lines in accordance withthe linefeed control signals 1012 provided by the signal processor.Linefeed control is provided in the form of control currents I1–I4.

Transistors Q1, Q4, and Q6 control the tip line. Transistors Q2, Q3, andQ5 control the ring line. During the on-hook and off-hook operationalstates, transistors Q1–Q6 are operated in their linear regions ofoperation. During the ringing state, however, the control signalsapplied by the signal processor force transistors Q1, Q6 and Q3, Q5 tooperate in cut-off and saturation regions of operation so that thesetransistors operate as switches or multiplexers selectively couplingeither V_(BAT) 1066 or ground 1070 to the tip 1080 and ring 1090 lines.An alternate pre-determined supply level 1072 may be used in lieu ofground 1070 to permit toggling between V_(BAT) and the alternate supplylevel 1072.

The direct drive method results in a differential ringing signalcomparable to that produced by traditional balanced or unbalancedringing. Similar to the traditional balanced ringing technique, however,this direct drive approach permits driving each of the tip and ringlines such that each provides approximately one-half of the entiresignal swing required for ringing. The direct drive method may be usedin conjunction with other waveforms to produce differential ringingsignals having non-sinusoidal shapes (e.g., trapezoidal, triangular,etc.).

Referring to FIG. 9, the illustrated small signal waveform 942 resemblesa rectified sine wave wherein rectification occurs at the sine wave meanvalue of zero. V₉₄₂=|B sin(wt)| where B is the sine wave peak voltage.The sine wave B sin(wt) is thus folded onto itself about the foldingpoint zero to produce |B sin(wt)|.

In one embodiment, V_(BAT) is of the form V_(BAT)=A|B sin(wt)|+D, whereD is the supply offset introduced by the power supply and A is theamplification factor of the power supply. The resulting V_(BAT) waveformis switched between the tip and ring lines to generate two waveforms forthe purpose of driving the tip and ring lines. The folded ringingtechnique may be generalized to allow for the ringing signal componentsto contribute a DC component to the differential ringing signal.

Referring to FIG. 9, any DC offset present in the small signal waveformV₉₄₂ is irrelevant and will not be amplified by the switching powersupply 930. Thus an input signal 942 of the form V₉₄₂=|B sin(wt)+C| orV₉₄₂=|B sin(wt)+C|−C will produce the same V_(BAT). A DC offset(independent of any supply offset provided by the power supply) can beintroduced into the differential ringing signal as a result of themodification to the AC component of the small signal waveform 942. Inparticular, modification of the AC component by selecting folding pointsother than the mean of the pre-folded AC component will introduce a DCcomponent into the differential ringing signal.

FIG. 11 illustrates one embodiment of a V_(BAT) waveform correspondingto a waveform, f(t), rectified or folded about a value other than itsmean, f(t). For periodic f(t) with period T,

${\overset{\_}{f(t)} = {\frac{1}{T}{\int_{0}^{T}{{f(t)}.}}}}\ $Generally, rectification of any waveform, f(t) about a value C can bedefined as follows:

${f_{RECT}(t)} = \left\{ \begin{matrix}{f(t)} & {{{for}\mspace{14mu}{f(t)}} \geq C} \\{C - {f(t)} + C} & {{{for}\mspace{14mu}{f(t)}} < C}\end{matrix} \right.$This rectified waveform is amplified and translated by the power supplyto produce V_(BAT). With a switching power supply, however, the extremesof F_(RECT)(t) correspond to the extremes of V_(BAT). Thus translationof f_(RECT)(t) by C to provide

${f_{RECT}(t)} = \left\{ \begin{matrix}{{f(t)} - C} & {{{for}\mspace{14mu}{f(t)}} \geq C} \\{C - {f(t)}} & {{{for}\mspace{14mu}{f(t)}} < C}\end{matrix} \right.$will have no effect on the power supply output V_(BAT). When C=0, theexpression for f_(RECT)(t) reduces to the well-known expression for arectified waveform:

$\left. {f_{RECT}(t)} \right|_{C = 0} = \left\{ {\begin{matrix}{f(t)} & {{{for}\mspace{14mu}{f(t)}} \geq 0} \\{- {f(t)}} & {{{for}\mspace{14mu}{f(t)}} < 0}\end{matrix} = {{f(t)}}} \right.$

Given that the power supply amplifies only the AC component of f(t),f_(RECT)(t) is referred to as “zero folded” whenever C= f(t) (i.e., themean of f(t)). For periodic f(t) of period T having the property thatf(t)− f(t)= f(t)−f(t+T/2), zero folding ensures that no DC component isintroduced into the differential ringing signal by f_(RECT)(t). Thiswould be the case, for example, with simple sinusoidal, trapezoidal,triangular, and sawtooth waveforms.

If f(t) is a sinusoid of period T, for example, zero-folding produces aV_(BAT) resembling a full-wave rectified sinewave of period T/2 and adifferential ringing signal of period T having no DC componentintroduced by the AC component of f_(RECT) (t). Zero-folding, however,is a special case that ensures that the same DC component is introducedinto both ringing signal components such that there is no DC componentin the differential ringing signal. A mismatch in the ringing signalcomponents can be used to deliberately create a non-zero differentialringing signal DC component distinct from any offset contributed by thepower supply. This mismatch is introduced through the use of non-zerofolding.

Although “folding” has been discussed with respect to the power supplyinput, the power supply output W(t) may be generated in a variety ofways. A folding line may be identified by a plurality of critical pointsat which the slope of the waveform W(t) abruptly changes sign. This linemay occur at a value D other than zero due to the introduction of anoffset by the power supply. D represents either a maximum or minimumextreme of W(t).

Referring to FIGS. 9 and 11, the signal processor controls the tipmultiplexer to couple the power supply to the tip line for the durationT1 while controlling the ring multiplexer to couple the ring line to thealternate source. The signal processor then controls the tip multiplexerto couple the tip line to the alternate source while controlling thering multiplexer to couple the ring line to the power supply output forthe duration T2. The period of the ring line, tip line, and differentialring signal waveforms is T=T1+T2. If T1=T2, then the period of V_(BAT)is T/2. If T1≠T2, then the period of V_(BAT) is T.

In the illustrated embodiment, at least one of the ringing signalcomponents is time-varying over the interval between any two criticalpoints. Thus for t∈(0,T2 ) there is some point t=z for which thederivative of at least one of L1(t) and L2(t) is non-zero. In otherwords, there exists some z∈(0,T2) such that either

$\left. {\frac{\mathbb{d}\;}{\mathbb{d}t}{{L1}(t)}} \middle| {}_{t = z}{\neq {0\mspace{14mu}{or}\mspace{14mu}\frac{\mathbb{d}\;}{\mathbb{d}t}{{L2}(t)}}} \middle| {}_{t = z}{\neq 0.} \right.$Similarly, there is some point z∈(0,T1) such that either

${\frac{\mathbb{d}\;}{\mathbb{d}t}\mspace{14mu}{{L1}(t)}{_{t = z}{\neq {0\mspace{14mu}{or}\mspace{14mu}\frac{\mathbb{d}\;}{\mathbb{d}t}\mspace{14mu}{{L2}(t)}}}}_{t = z}} \neq 0.$

FIG. 12 illustrates a generalized method for generating a differentialringing signal between the tip and ring lines. A power supply is coupledto a linefeed driver for driving the tip and ring lines. During theon-hook and off-hook states, the linefeed driver is operated as a linearamplifier. During the ringing state, however, the linefeed driver isoperated as switching circuitry. The active devices of the linefeeddriver are controlled to selectively couple the tip and the ring linesto either the power supply or an alternate source.

In step 1210, a first waveform exhibiting rectification is provided.Rectification is typified by a plurality of critical points located at aminimum or maximum extreme of the waveform. These critical points followa line of folding that represents folding of a waveform about a foldingpoint. Although rectification is typically presumed to be about thefolding point zero such that F_(RECT)(t)=|f(t)|, rectification may occurabout a non-zero rectification point such that f_(RECT)(t)=|f(t)−C|+C,where C is non-zero. In various embodiments, f(t) is a sinusoidal ortrapezoidal waveform. In one embodiment, f(t) is periodic with period Tand “balanced” in time such that f(t)=−f(t+T/2).

FIG. 13 illustrates rectification of a variety of waveforms about theirmeans or values other than their means. Sinusoid waveform (a) isillustrated without any DC offset and thus has a mean value of zero.Waveform (b) illustrates rectification of sinusoid (a) about 0 (foldingpoint 1312). The folding operation results in a plurality of criticalpoints 1314 lying on the same folding line 1316 defined by the foldingpoint 1312. In the illustrated embodiment, the critical points ofwaveform (b) are equidistant. The period of waveform (b) is half that ofthe period of waveform (a).

Waveform (c) illustrates folding waveform (a) about a folding pointother than zero. Although the mean is zero, the folding point 1322 isless than zero (i.e., C<0). A plurality of critical points 1324 isexhibited along the folding line 1326 defined by the folding point 1322.In this case, the critical points are not equidistant. Moreover, theperiod of waveform (c) is the same as that of waveform (a). Given aduration T1 between a first and an adjacent second critical point on thefolding line 1326 and a duration T2 between the second and the nextadjacent critical point along the folding line, the period T of thefolded sinusoid is calculated as T=T1+T2. Clearly, for a waveform havingmore critical points within the period, T may be decomposed into moreindividual components, T1, T2, . . . T_(n).

Waveform (d) illustrates a trapezoidal waveform comprising criticalpoints before rectification. Waveform (d) also exhibits a non-zero meanor DC offset. Waveform (e) illustrates folding waveform (d) aboutfolding point 1342 to generate a folded trapezoid waveform. In thisexample, critical points 1330 of the trapezoidal waveform (d) arepreserved through rectification. Rectification generally increases thenumber of critical points in the waveform being rectified.

Waveform (d) has a mean of C. Folding waveform (d) about C produces arectified waveform (e) that exhibits a period one-half that of waveform(d). Rectification introduces additional critical points 1344 along thefolding line 1346 defined by the folding point 1342. Although switchingoccurs near critical points for the ringing application, the switchingis controlled by the critical points 1344 along the folding line 1346rather than other critical points 1330.

Waveform (f) illustrates folding a trapezoidal waveform having a DCoffset about a folding point 1352 other than the DC offset. Theresulting waveform exhibits non-equidistantly spaced critical points1354 along the folding line 1356 defined by the folding point 1352.

Waveform (g) illustrates folding a triangular waveform having a DCoffset about a folding point 1362 other than the DC offset. The criticalpoints 1364 occurring along the folding line 1366 are non-equidistantlyspaced.

Referring back to step 1210 of FIG. 12, the folded nature of therectified waveform results in a periodic series of critical points. Theduration between a first critical point and a second critical pointalong the folding line is T1. The duration between the second criticalpoint and a third critical point is T2. A period, T, of the rectifiedwaveform in this case does not exceed T1+T2, i.e., T≦T1+T2.

A translated AC component of the rectified waveform is amplified togenerate the power supply output, V_(BAT) in step 1220. V_(BAT) may alsoinclude a supply offset, D, such thatV_(BAT)=A(F_(RECT)(t)−C)+D=A(|f(t)−C|)+D.

When in the ringing state, the linefeed driver behaves as switchingcircuitry to switch the tip and ring lines between a first state and asecond state. In the first state, the switch circuitry is controlled tocouple the tip line to the power supply while coupling the ring line toan alternate source. In the second state, the switch circuitry iscontrolled to couple the tip line to the alternate source while couplingthe ring line to the power supply output.

The switch circuitry is toggled from one state to a distinct other stateeach time V_(BAT) exhibits a critical point. In one embodiment, V_(BAT)is determined to be near a critical point when a magnitude of V_(BAT) isless than or equal to a pre-determined switching threshold, K (i.e.,|V_(BAT)|≦K).

Step 1230 determines if |V_(BAT)|≦K. If not, the switches remain intheir current state. Once |V_(BAT)|≦K (i.e., indicating that a criticalpoint is near or has been reached), step 1240 toggles the switches tothe first state in which V_(BAT) is applied to the tip line while analternate source, V_(ALT) is applied to the ring line. Step 1250 isprovided to prevent re-toggling of the switches near the currentcritical point by disabling further processing until |V_(BAT)|>K again.Once |V_(BAT)|>K, the conditions for toggling do not occur until thenext critical point and processing continues with step 1260.

Step 1260 determines whether the next critical point has been reached.The switches remain in their current state until |V_(BAT)|≦K. Once|V_(BAT)|≦K (i.e., indicating that the next critical point has beenreached), step 1270 toggles the switches to the second state in whichV_(BAT) is applied to the ring line while V_(ALT) is applied to the tipline. Step 1280 is provided to prevent re-toggling of the switches nearthis inflection point by disabling further processing until |V_(BAT)|>Kagain. Once |V_(BAT)|>K, the conditions for toggling do not occur untilthe next critical point. The process continues by repeating steps1230–1280.

The result is that the switch circuitry couples V_(BAT) to the tip linewhile coupling V_(ALT) to the ring line upon reaching a first V_(BAT)critical point and toggling to the first state. The switch circuitryremains in the first state for the duration T1 until a second V_(BAT)critical point is reached. Upon reaching the second V_(BAT) inflectionpoint, the switch circuitry is toggled to the second state in whichV_(BAT) is coupled to the ring line while V_(ALT) is coupled to the ringline. The switch circuitry remains in the second state for the durationT2 until the next V_(BAT) critical point is reached and the processrepeats by toggling the switch circuitry back to the first state. In oneembodiment, V_(ALT)=0 such that the alternate source is ground. Inanother embodiment, the alternate source has a same value as the supplyoffset such that V_(ALT) is not grounded (i.e., V_(ALT)=D).

The period of the resulting differential ringing signal as well as thetip and ring components of the differential ringing signal is T=T1+T2.When T1≠T2, the period of V_(BAT) is also T. If on the other hand, T1=T2then the period of V_(BAT) is T1=T2=T/2.

When T1≠T2, the method of generating the differential ringing signalintroduces a DC offset into the differential ringing signal. This DCoffset is distinct from any supply offset, D, contributed by the powersupply. Indeed given that D is introduced to both the tip and ringlines, D will have no effect on the differential ringing signal ifV_(ALT)=D. The DC offset in the differential ringing signal is afunction of the folding point about which a non-rectified waveform isfolded to generate V_(BAT) as well as the shape of the non-rectifiedwaveform itself.

Mathematically, the differential ringing signal is derived from thetime-varying power supply of which the differential ringing signal is acomponent. Note that the expression for a rectified waveform includesthe desired differential ringing signal. In particular, a time-varyingpower supply of the form W(t)=|f(t)−C|+C+D.

Referring to FIG. 9, various components of the SLIC may be incorporatedinto one or more integrated circuit packages. In one embodiment, thesignal processor and portions of the switching power supply 930 areincorporated onto one or more semiconductor dice within a commonintegrated circuit package. In one embodiment, for example, signalprocessor 910, and portions of the power supply 930 including comparator940 and saw tooth generator 944 are fabricated on a same semiconductorsubstrate 970 within a single integrated circuit package. In oneembodiment, these components are implemented as a complementary metaloxide semiconductor (CMOS) integrated circuit.

When operated as switching circuitry rather than a linear amplifier, thelinefeed driver consumes little power and therefore dissipates littleheat. Thus switching circuitry 920 is suitable for fabricating on asemiconductor substrate of an integrated circuit.

The linefeed driver 920 may be implemented as discrete components asillustrated in FIG. 10. Alternatively, at least the power portion of thelinefeed driver may be implemented as an integrated circuit. The powerportion of the linefeed driver may be fabricated as a high voltageintegrated circuit residing on a common semiconductor substrate within asingle integrated circuit package.

Methods and apparatus for generating a ringing signal have beendescribed. A power supply generates a rectified waveform. The rectifiedwaveform has an AC component corresponding to a balanced waveform foldedabout a folding point. The power supply output waveform has a pluralityof critical points along the folding point. The supply output isprovided to a linefeed driver operating in a switch mode when in aringing state. The linefeed driver switching effectively decomposes thepower supply waveform into a plurality of waveforms for application tothe tip and ring lines. The linefeed driver switching is toggled by thecritical points along the folding line defined by the folding point.

Upon encountering a critical point, the switches are toggled to a firststate to couple the power supply to the tip line while applying analternate source to the ring line. The switches remain in the firststate for the duration T1 until encountering the next critical pointlying on a same folding line. The switches are then toggled to a secondstate to couple the power supply to the ring line while applying thealternative source to the tip line. The switches remain in the secondstate for the duration T2 until encountering the next critical point.The period of the differential ringing signal is T1+T2=T.

When the folding point is zero and the waveform has the property thatV(t)=−V(t+T/2) before rectification, the critical points used fortoggling are equidistant. The power supply provides a waveform having aperiod T/2 that substantially resembles a full-wave rectified waveform.The waveforms appearing on the tip and ring lines resemble half-waverectified waveforms having a 180° relative phase difference and a periodT. No DC offset is introduced into the differential ringing signal. Thus∫₀ ^(T)ΔL(t)≈0.

When the folding point is nonzero, T1≠T2, the power supply outputwaveform provides a periodic waveform having a period T=T1+T2. Thecritical points used for toggling are not equidistant, thus resulting ina DC offset introduced into the differential ringing signal. Thus ∫₀^(T)ΔL(t)≠0.

Examined from another perspective, the ringing signal components L1(t),L2(t) are derived from the time varying power supply W(t)=|f(t)−C|+C+D,where D is a DC offset contributed by the power supply. C is a foldingline about which f(t) is folded. The differential ringing signalΔL(t)=L1(t)−L2(t). The signal processor controls the linefeed driver totoggle between 1) coupling W(t) to L1 (e.g., the tip line) whilecoupling L2 (e.g., the ring line) to an alternate supply, V_(ALT)(t),and 2) coupling W(t) to L2 while coupling L1 to V_(ALT)(t).

The toggling occurs when W(t)≦K, wherein K is a pre-determined switchingthreshold near critical points of W(t). K is selected such that a firsttoggling occurs at W(t₁+ε₁) and a second toggling occurs at W(t₂+ε₂),wherein W(t₁) and W(t₂) are critical points of W(t). In one embodiment,|ε₁|, |ε₂| are small relative to the time Δt between t₁ and t₂. Inparticular, |ε₁|, |ε₂|<<Δt=|t₁−t₂|.

A DC bias, E, can be introduced into the differential ringing signalsuch that ΔL(t)=f(t)+E from a time-varying supply level of the formW(t)=|f(t)−C|+C+D. For f(t) of period T with the propertyf(t)=−f(t+T/2), this is readily achieved by selecting a non-zero C

In the preceding detailed description, the invention is described withreference to specific exemplary embodiments thereof. Variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the invention as set forth in the claims.The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A method of generating a subscriber line ringing signal for asubscriber line having first and second lines, comprising: a) applying atime-varying supply level W(t)=|f(t)−C|+C+D to the first line whileapplying an alternate source V_(ALT)(t)=D to the second line whenf(t)−C>0, wherein D is a supply level DC offset, wherein C is a foldingline about which f(t) is folded, wherein C≠0; and b) applying thetime-varying supply level to the second line while applying thealternate source to the second line when f(t)−C≦0, wherein a resultingringing signal component of the first line is L1(t), wherein a resultingringing signal component of the second line is L2(t), wherein the firstand second lines form a differential ringing signal line pair providingthe differential ringing signal ΔL(t)=L1(t)−L2(t)=f(t).
 2. The method ofclaim 1 wherein D=0.
 3. The method of claim 1 wherein f(t) is periodicwith period T, wherein${\frac{1}{T}{\int_{0}^{T}{\Delta\;{L(t)}}}} = {\overset{\_}{L(t)} \neq 0.}$4. The method of claim 1 wherein W(t)=L1(t)+L2(t).
 5. The method ofclaim 1 wherein steps a) and b) are initiated near critical points ofW(t) when W(t)−K=0, wherein K is a pre-determined switching threshold,wherein step a) is initiated near a first critical point W(t₁) atW(t₁+ε₁), wherein step b) is initiated near a subsequent second criticalpoint W(t₂) at W(t₂+ε₂), wherein |ε₁|, |ε₂|<<Δt=|t₁−t₂|.
 6. An apparatusfor generating a subscriber line ringing signal, comprising: a powersupply providing a time-varying supply level W(t)=|f(t)−C|+C+D, whereinD is a power supply offset, wherein C≠0; a linefeed driver; and a signalprocessor, wherein when W(t)≦K the signal processor controls thelinefeed driver to toggle between 1) coupling W(t) to a tip line whilecoupling a ring line to an alternate supply, V_(ALT)(t), and 2) couplingW(t) to the ring line while coupling the tip line to V_(ALT)(t), whereinK is a pre-determined switching threshold.
 7. The apparatus of claim 6wherein D=0.
 8. The apparatus of claim 6 wherein f(t) is periodic withperiod T, wherein${\frac{1}{T}{\int_{0}^{T}{\Delta\;{L(t)}}}} = {\overset{\_}{L(t)} \neq 0.}$9. The apparatus of claim 6 wherein K is selected such that the togglingoccurs near critical points of W(t), wherein a first toggling occurs atW(t₁+ε₁), wherein a second toggling occurs at W(t₂+ε₂), wherein W(t₁)and W(t₂) are critical points of W(t), wherein |ε₁|, |ε₂|<<Δt=|t₁−t₂|.10. A method of generating a differential ringing signal with a DCcomponent between a tip and a ring line of a subscriber line,comprising: a) providing a time-varying supply level, W(t), having aplurality of critical points along a folding line, wherein the criticalpoints are substantially not equidistant; b) coupling W(t) to the tipline while coupling an alternate source to the ring line in response toa first critical point; and c) coupling W(t) to the ring line whilecoupling the alternate source to the tip line in response to a secondcritical point.
 11. The method of claim 10 wherein the differentialringing signal has a period T, wherein a duration between the first andsecond critical points is T1, wherein a duration between the second anda next critical point is T2, wherein T≧T1+T2.
 12. The method of claim 10wherein T1≠T2, wherein a period of W(t) is T=T1+T2.
 13. The method ofclaim 10 wherein the differential ringing signal is one of a sinusoidal,a trapezoidal, a sawtooth, and a triangular waveform.
 14. An apparatusfor generating a subscriber line differential ringing signal having a DCcomponent between a tip line and a ring line, comprising: switchcircuitry coupling the tip line to a time-varying power supply W(t)having a plurality of non-equidistantly spaced critical points whilecoupling the ring line to an alternate source when in a first state,wherein the switch circuitry couples the tip line to the alternatesource while coupling the ring line to W(t) when in a second state; anda signal processor toggling the switch circuitry between the first andsecond states in response to the critical points.
 15. The apparatus ofclaim 14 wherein the switch circuitry is toggled to the first state inresponse to a first critical point, wherein the switch circuitry istoggled to the second state in response to a second critical point. 16.The apparatus of claim 15, wherein a duration between the first andsecond critical points is T1, wherein a duration between the secondcritical point and a next critical point is T2, wherein the differentialringing signal has a period T≧T1+T2.
 17. The apparatus of claim 16,wherein the differential ringing signal has a period T=T1+T2.
 18. Theapparatus of claim 14 wherein W(t) resembles one of a full-waverectified sinusoidal and a full-wave rectified trapezoidal waveform. 19.A method of generating a differential ringing signal, comprising: a)applying a ringing signal component L1(t) to the tip line; and b)applying a ringing signal component L2(t) to the ring line, whereinL2(t)≠L1(t+T/2), wherein L1(t) and L2(t) have a period of T, wherein atleast one of L1(t) and L2(t) varies over the interval t∈(0,T/2), whereinthe differential ringing signal ΔL(t)=L1(t)−L2(t).
 20. The method ofclaim 19 wherein step a) further comprises: i) applying a time-varyingsupply level W(t) to the tip line for a duration T1; and ii) applying analternate supply level to the tip line for a duration T2, wherein T1≠T2.21. The method of claim 20 wherein step b) further comprises: i)applying the alternate supply level to the ring line while applying W(t)to the tip line for the duration T1; and ii) applying W(t) to the ringline while applying the alternate supply level to the tip line for theduration T2.
 22. The method of claim 20 wherein step i) is initiatedwhen W(t) is near a first critical point, wherein step ii) is initiatedwhen W(t) is near a subsequent second critical point.
 23. An apparatusfor generating a subscriber line ringing signal, comprising: a powersupply providing a time-varying supply level W(t) having a plurality ofnon-equidistantly spaced critical points along a same folding line; alinefeed driver; and a signal processor controlling the linefeed driverto couple W(t) to a tip line while maintaining a ring line at apre-determined supply level when in a first state, wherein the signalprocessor controls the linefeed driver to couple W(t) to the ring linewhile maintaining the ring line at the pre-determined supply level in asecond state, wherein a resulting ringing signal component of the tipline is L1(t), wherein a resulting ringing signal component of the ringline is L2(t), wherein a differential ringing signal ΔL(t)=L1(t)−L2(t)has a period T.
 24. The apparatus of claim 23 wherein the differentialringing signal is one of a sinusoidal, a trapezoidal, a sawtooth, and atriangular waveform.
 25. The apparatus of claim 23 wherein the signalprocessor toggles the linefeed driver between the first and secondstates in response to the critical points.
 26. The apparatus of claim 23wherein the coupling of W(t) to a selected one of the tip and ring linesis initiated when |W(t)|≦K, wherein |W(t)| is an absolute value of W(t),wherein K is a pre-determined switching threshold.
 27. The apparatus ofclaim 23 wherein the pre-determined supply level is ground.
 28. Theapparatus of claim 23 wherein L1(t) and L2(t) resemble one of ahalf-wave rectified sinusoidal and a half-wave rectified trapezoidalwaveforms.
 29. The apparatus of claim 23 wherein W(t) resembles one of afull-wave rectified sinusoid and a full-wave rectified trapezoid.